Motor vehicles, as presently manufactured, are equipped with systems for defogging and deicing windshields. Generally, these systems depend upon heat generated in the internal combustion engine and transferred to the engine's cooling system to be blown as warm air across the interior of the windshield to accomplish the defogging and deicing. In such a case, of course, it is readily apparent that there is a period of time between the starting of an engine and the time that sufficient heat is being generated in its cooling system in order to provide a defogging and deicing of the vehicle's windshield. Depending upon the exact temperature conditions and the time the vehicle has been sitting idle without its engine running, the period of time before sufficient heat is available to accomplish this function can be up to 10 minutes or more.
In view of the fact that there can be a rather lengthy delay before the present day motor vehicles heating and defrosting system can clear a windshield, automotive designers have been attempting to design systems which generate heat from electrical energy to accomplish a relatively rapid defrost and deicing of a vehicle windshield. Designers have also been attempting to design such an electrically heated vision unit in which heat can be concentrated in a selected zone or zones thereof which will initially be defogged and deiced. For example, areas desired to be more rapidly defogged and deiced may be those located directly in front of the driver and at opposite edges of the vision unit if such a vision unit is a windshield. Such rapid defrosting and deicing systems utilizing electrical energy generally are independent of the normal heating and defrosting system contained in a motor vehicle.
Many different systems have been proposed for accomplishing this rapid defrost and deicing function, including the placement of an electrically conductive transparent coating on the windshield and embedding of fine wires in a laminating interlayer of the windshield. To the best of our knowledge, there are no such rapid defrost and deicing system in vehicles which are currently sold in the U.S. market. We believe this is because of the relatively high cost of such systems and also because of the electrical problems associated with installment of such a system on a vehicle windshield such as found in today's automobiles. The windshield found in today's automobile is generally of trapezoidal shape. By this we mean the modern day windshield is smaller in length dimension at the top thereof than at the bottom thereof. The top dimension of the windshield is reduced because the aerodynamic styling of motor vehicles dictates such a configuration for the windshield to fit on the rounded body shapes now being manufactured.
One of the inventors of the subject matter of this disclosure, namely, Kevin J. Ramus, has issued on Sept. 24, 1985, U.S. Pat. No. 4,543,466, entitled "Bus Bar Arrangement for Uniformly Heating a Trapezoidally Shaped Electrically Heated Windshield." Briefly, this patent proposes a system for applying a generally uniform heating of a trapezoidally shaped electrically heated windshield.
In particular, the patent discloses an electrically heated windshield of generally trapezoidal shape having bus bars of uniform conductivity throughout their length extending in line contact with the upper and lower edges of a continuous uniformly thick trapezoidally shaped electrically conductive coating provided on the windshield surface. This construction is used for deicing and defrosting the windshield. The line of contact of the upper bus bar with the conductive coating has a length generally equal to the entire effective length of the upper edge of the conductive coating. The lower bus bar is symmetrically located along the lower edge of the conductive coating and has a line of contact length equal to the sum of the length of the upper bus bar plus generally about one-half of the difference between the entire effective length of the lower edge of the conductive coating minus the length of the line of contact of the upper bus bar. This structure, however, does not teach an electrically heatable vision unit in which heat can be concentrated in a selected zone or zones thereof as is taught in the specification.
Mr. Ramus also personally conducted a search in the U.S. Patent and Trademark Office to determine if the electrically heatable vision unit as taught in this specification was novel. During his search, he uncovered only one patent which was felt to be of interest with respect to the subject matter of this specification. The patent uncovered was U.S. Pat. No. 3,621,441, which issued on Nov. 16, 1971, for "Film Resistor Adjustable by Isolating Portions of the Film."
This patent teaches a film resistor which includes a pair of conductive bus bars which are spaced apart from each other and are deposited adjacent to opposite edges of one surface of a substrate. A plurality of spaced parallel fingers extend transversely from each of the conductive bus bars and partially into the space between the conductive bus bars on the substrate. A resistive film is deposited onto the substrate in the space between the conductive films and overlaps the free ends of the fingers. The resistance value of the film resistor can be adjusted by removing selectively some or all of the fingers so that adjacent portions of the resistive film are substantially isolated electrically when the resistor is used in an environmental circuit even though the entire resistive film remains attached physically to the substrate and unmarred. From the following specification, it will be obvious that this structure is different than our structure proposed for an improved electrically heatable vision unit in which heat can be concentrated in a selected zone or zones thereof.